This invention relates to apparatus for detecting the presence of or counting articles being conveyed in a stream. The invention has particular application to the graphic arts industry wherein printed articles such as newspapers, books, magazines and the like are conveyed from one operation to another.
Heretofore counters employed in the graphic arts industry have generally required a physical contact with the printed product, often requiring the stream to be confined between upper and lower conveyor bands. The contact may be a mechanical sensor engaged by the leading edge of the article, or a guide member for positively guiding the article past a photo-electric sensor. Irregularities in the product, such as newspapers, and the dynamics of the sensor and the article at high speed printing operations can lead to erroneous and inaccurate counting. Moreover, where high quality signatures are produced, physical contact with the surface of the article can damage the products.
The J. A. Stegenga U.S. Pat. No. 3,414,732 discloses a counter which does not require physical contact with the articles, and does not require a confined stream. A light source is projected onto the stream of products to illuminate the products for an electro-optical sensor. The latter registers the passage of an article by detecting the shadow, or change in illuminance created by the leading edge of the article. However, to reduce the possibility of the sensor reacting to a densely printed area on the product, a high intensity light source is required in conjunction with a relatively long, narrow slit in the sensor housing to sense a substantial area of the product. Additionally, a reflective plate is required to be mounted below the stream to maintain the normal illuminance when no product is present in the stream, whereas without the plate, a lessening of the illuminance would cause a count pulse to be generated. The Stegenga device also requires an air supply to maintain the long, narrow slit free of dust accumulation.